They make passive subs for both ported and sealed. For example, the RBH SI-1212 and Unrivaled-21 subs are sealed passive subs.
Just not passive for “servo”, which is a gimmick to me. I used to own dual servo subs.
Servo isn't a gimmick and not it's only used for subwoofer applications, FWIW. I believe what
@TLS Guy was saying is , and could be mistaken, that amplification for sealed subs has to have the ability, whether DSP or fixed options, to taylor sound correctly, including extension, meaning you can't just plug in any amp and achieve the desired results. That can happen with ported subs...